Slurry casting is one of the most frequently used method for forming ceramic articles. Its primary advantages include ease of operation, low cost and wide applicability. The method can be used for producing numerous ceramic articles of complex shapes and sizes.
Earlier slurry casting machines, however, suffer from disadvantages in low precision, poor homogeneity and poor reproducibility. In view of these and other problems, pressurized slurry casting machines have been under development.
Conventional pressurized slurry casting machines for ceramic articles, nevertheless, has some shortcomings such as poor air tightness in the feed tank, difficulty for vacuum operation and frequent plugging by agglomerates of materials in the flow conduit.
With reference to FIG. 4, the operation of a conventional pressurized slurry casting machine for ceramic articles will be illustrated. FIG. 4 is representative of a present pressurized slurry casting machine. With valves 401 and 901 closed, the feed tank 10 and the transport conduit 40 are first vacuumed by the vacuum system 102, after which the vacuum system 102 is turned off and the feed valve 901 is opened. The slurry feed 90 is then drawn rapidly into feed tank 10 by vacuum suction. After the desired amount of feed enters the feed tank 1, the tank is further vacuumed by turning on again vacuum system 102 while the feed is agitated by a propeller 201 fitted on an agitator shaft 20 driven by motor 30 such that air bubbles in the slurry feed is expelled from the slurry (degassing). After sufficient degassing, the vacuum system 102 is turned off again and the pressurizing system 101 and control valve 401 are turned on such that the slurry feed 90 is pressurized and thus forced into mold 70 by way of transport conduit 40. At the same time the pressing plate 60 is lowered to exert pressure against mold 70. After consolidation of the slurry feed in the mold, the product is demouled. The apparatus is then cleaned for next production.
Such conventional pressurized slurry casting apparatus has many disadvantages, some of which are illustrated below:
(1) The horizontal arrangement and the multiple elbows of the transport conduit 40 render the degassing of the slurry within the section from propeller 201 to control valve 40 nearly impossible. As a consequence, the slurry will unavoidably contain a certain amount of bubbles after entering mold 70 and the quality of the product will be significantly deteriorated by the existence of bubbles in the final product. PA0 (2) Precipitation in the slurry feed and plugging by the slurry are frequently observed in the transport conduit 40 due to the horizontal arrangement and multiple elbows of the transport conduit 40. Such phenomena frequently result in difficulties in operation and homogeneity problem in the products. PA0 (3) During the casting of the slurry, if the slurry feed is not sufficient, air will enter the mold through the space on the upper part of the transport conduit 40 as shown by FIG. 4 and result in air bubble defects in the product. PA0 (4) After the casting operation, the residual slurry in the conduit 40 can not be easily removed and will result in waste of the raw material. Furthermore, the residual slurry will create severe plugging problem in the conduit if the residual slurry solidified in the conduit. Due to the horizontal arrangement and elbows of the conduit, it is rather difficult to clean the conduit after casting. If the apparatus is not thoroughly cleaned for next operation, old slurry will mix with new slurry and result in unstable and poor quality of the final product. PA0 (5) No vacuum system is used for the casting system 50 and therefore the air in the mold will constitute a resistance to the entrance of the slurry feed. As a result the mold cavity cannot be completely filled by the slurry feed and the final product will unavoidably contain bubbles and the failure rate will be increased. PA0 (6) Air tightness of the bearing section of the agitator shaft which extends from outside of the feed tank frequently adds to the difficulties in vacuuming the feed tank 10. Furthermore, the metal scraps abraded off by the propeller under high power rotation will form contaminations of the feed and result in speck defects in the final product. PA0 (1) a feed tank fitted with an agitator, the propeller of the agitator being at just above the bottom of the feed tank; PA0 (2) a vertical conduit comprising a lower opening positioned above the agitator and an upper opening for delivering the slurry to a mold, the conduit being connected with the feed tank in an air tight relation except in the openings of the conduit; PA0 (3) means for fixing the mold above the upper opening of the conduit with the mold cavity of the mold connected with the vertical conduit in an air tight relation; PA0 (4) a pressurizing system capable of applying positive pressure in the feed tank; and PA0 (5) a vacuum system capable of applying negative pressure in the feed tank.
In the conventional pressurized slurry casting, waste of material is frequently encountered. The products of such conventional casting machine often exhibits pin holes and specks of impurities, rendering their quality unstable.
With these and other drawbacks, conventional pressurized slurry casting machines for ceramic articles are used solely for conventional ceramic products but not for the production of fine ceramics. Accordingly, there is need for a satisfactory improvement to this type of casting machine.